Re: Burning EPROMs

[mjmahon@aol.com (Michael J. Mahon)]

Stephen Shaw replied:

>On Mon, 16 Dec 2002 21:11:22 +0000, Michael J. Mahon wrote:
>
>> Stephen Shaw wrote:
>> 
>>>On Sun, 15 Dec 2002 16:13:45 +1300, David Empson wrote:
>>>
>>>> Herbert Fung <lemonherb1@netscape.net> wrote:
>>>> 
>>>>> Hi, I was wondering if there is any specific type of EPROM one would
>>>>> have to get to burn repacements for various Apple II related cards.
>>>> 
>>>> It depends on the card.  Older cards (or simpler ones) might use EPROMs
>>>> as small as the 24-pin 2708 (1KB).  The 24-pin 2716 (2KB) and 2732/2532
>>>> (4KB) were also commonly used on older cards.
>>>
>>>2708s are obsolete and most EPROM programmers that can handle the 2716s
>>>upwards do not seem to handle the 2708 (different programming voltage?).
>>>I have a lot of Epson APL printer cards minus their 2708s :(
>> 
>> You can substitute the much more popular 2716, perhaps with one circuit
>> mod.
>> 
>> The only difference in pinout is that:  1) pin 18 is the "program" control
>> line on the 2708, but is /CE on the 2716.  If this is kept at logic low by
>> the card, it's fine.  2) Pin 19 is Vdd (the negative supply) on the 2708,
>> but is A10 for the 2716.  You will probably need to cut the trace to this
>> pin and ground it.  This will result in locking the EPROM to the lower 1KB
>> of its address range, which is where the firmware should go. 
>> (Alternatively, you could tie pin 19 to A10 on the card bus connector, and
>> have a 2KB EPROM space to play with. ;-)
>> 
>>>> More recent cards tended to use larger EPROMs even if they didn't need
>>>> the extra space, because the EPROMs were easier to burn and cheaper.
>>>> The 28-pin 2764 (8KB) and 27128 (16KB) were used on several cards,
>>>> including the original Apple SCSI card.  The 27256 (32KB) may have been
>>>> used on cards that needed a lot of firmware (with a significant amount
>>>> of bank switching or an onboard CPU).
>>>> 
>>>> It is usually possible to replace a 28-pin EPROM with one of larger
>>>> capacity, but it depends on the signals that are connected to the extra
>>>> address pins.  To be on the safe side, you should try to use a
>>>> replacement ROM of the same type as the original.  If you only have a
>>>> larger capacity ROM available, then you have the best chance of it
>>>> working by duplicating the contents of the smaller ROM into both halves
>>>> (or all quarters or eighths) of the larger ROM.
>> 
>> -michael
>> 
>
>Thanks. I'll save this one for when I get a chance to tackle my APL cards.

Stephen, I overlooked one pin:  pin 21 of the 2708 is -5v, and this pin is Vpp
on the 2716, so the trace to this pin should also be cut and it should be
grounded.

-michael

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